The present invention relates to a process for hydriding halogen-substituted compounds of elements of the 2nd to 4th period of Groups III and V of the periodic table of elements with the exception of gallium, carbon, aluminum, and nitrogen.
For the synthesis of covalent hydrides of the elements silicon, germanium, and boron, the reaction of the corresponding halides with the ionic hydrides of the alkali metal and alkaline earth metals in molten salt has been described in the past. See German Patent 10 80 077; W. Sundermeyer, O. Glemser, Angew. Chemie 70 (1958) 625; H.-H. Emons et al., Z. Chem. 23 (1983) 349.
In selecting the components for the formation of the molten salt bath, care must be taken that the melt is not reduced as a result of reaction with the hydrides of the alkali metal and alkaline earth metals. Furthermore, the temperatures used in carrying out the reaction have an upper limit as a result of the incipient thermal decomposition of the hydrides of boron, silicon, and germanium. Thus, thermal decomposition begins at 400.degree. C. in the case of silane, and, in the case of germanium, 280.degree. C.
If, under these constraints, consideration is given to the possible combinations of salts, whose components are stable in the presence of alkali metal and alkaline earth metal hydrides, and whose eutectic is at a maximum of 350.degree. C., it is apparent that only mixtures of salts that contain lithium chloride meet these criteria.
However, the use of a molten salt melt containing LiCl has a significant disadvantage. In the course of the reaction of the alkali metal or alkaline earth metal hydride with the halides of silicon, germanium, boron, etc. to form the corresponding hydrogen compounds and with the simultaneous formation of the halides of the alkali metal or alkaline earth metals, the composition of the molten melt changes. Therefore, extra addition of individual components of the melt is necessary in order to keep the composition of the melt constant. This leads to a corresponding increase in the volume of the melt so that, in order to avoid overfilling the reactor a portion of the melt must be drained off. The draining can take place either constantly or at intervals.
The LiCl which appears in the waste sediment has a high value and therefore necessitates expensive processing for recovery.
H. L. Jackson et al. (Inorg. Chem. 2 (1963) 43) describe a process in which silicon tetrachloride and dimethyldichlorosilane are caused to react with hydrogen to form silane in a NaCl-AlCl.sub.3 eutectic melt in the presence of aluminum under extremely high pressures (several hundred bars) and with long reaction times. Details pertaining to yields are not included in this publication, but it is said that as a result of using dimethyldichlorosilane only low conversion is achieved.
The aim of the invention is to provide a process by means of which it is possible to hydride halogen-substituted compounds of various elements with high yields in molten salt melts that contain no lithium chloride.